The recent trend of thin flat panels is in large size. Especially, in the field of television, even a liquid crystal panel of more than 100 inches in size appears, and it is thought that this trend will remain unchanged in the future. A thin film transistor liquid crystal display (TFT_LCD) panel is provided with data lines. The data lines are driven by amplifiers of an LCD driver. According to increase in size of the liquid crystal display panel, the load of each of the data lines becomes heavier, and thus a power consumption amount in the amplifier tends to increase. Further, in order to reduce the number of LCD drivers to be used, the number of outputs of one chip tends to increase. As a result, the power consumption amount per one chip increases. This causes a problem that the power consumption amount of the whole of LCD drivers increases, resulting in an abnormal elevation in chip temperature.
As a measure against the elevation in chip temperature, a system draws attention, in which an intermediate voltage VDD/2 which is a half of a high-voltage side power supply voltage VDD is supplied to a chip, and an operational amplifier operating in this voltage is used to reduce power consumed by the chip. In accordance with this system, however, various problems in a circuit arise. For example, if the chip is merely driven with the power supply voltage of VDD/2, the voltage range of the operational amplifier is limited, so that a positive side operational amplifier operates in the range from the intermediate voltage VDD/2 to the high-voltage side power supply voltage VDD, and an negative side operational amplifier operates in the range from the low-voltage side power supply voltage VSS (GND) to the intermediate voltage VDD/2. For this reason, a switch needs to be connected to an output of the operational amplifier for polarity inversion. However, the switch is very large in size, causing increase in chip area. Furthermore, there is such a problem that an on-resistance of the switch causes dullness in output waveform.
This problem will be described with reference to an operational amplifier described in patent literature 1 as an example. FIG. 1 shows a configuration of a conventional operational amplifier. The conventional operational amplifier is provided with differential input stage circuits 140 and 240 which are supplied with a high-voltage side power supply voltage VDD and a low-voltage side, power supply voltage VSS, and driving stage circuits 130 and 230, switch circuits 300, 400, 500, and 600, P-channel MOS transistors MP180 and MP280 (hereinafter, to be referred to as “transistors MP180 and MP280”), and N-channel MOS transistors MN180 and MN280 (hereinafter, to be referred to as “transistors MN180 and MN280”).
The driving stage circuit 130 is connected to an output terminal 110 via the drains of the transistors MP180 and MN180. Similarly, the driving stage circuit 230 is connected to an output terminal 210 via the drains of the transistors MP280 and MN280. The source of the transistor MP180 is supplied with the high-voltage side power supply voltage VDD, and the source of the transistor MN180 is supplied with an intermediate voltage between the high-voltage side power supply voltage VDD and the low-voltage side power supply voltage VSS, namely, an intermediate voltage VDD/2 which is ½ of the high-voltage side power supply voltage VDD. Further, the source of the transistor MP280 is supplied with the intermediate voltage VDD/2, and the source of the transistor MN280 is supplied with the low-voltage side power supply voltage VSS.
The switch circuit 300 is provided with switches SW301 to SW304 to control connections between the output terminals 110 and 210 and an odd terminal 310 and an even terminal 320. The switch circuit 400 is provided with switches SW401 to SW404 to control connections between terminals 410 and 420 and input terminals 120 and 220 of the differential input stage circuits 140 and 240. Here, a positive-polarity voltage INP is inputted from a positive digital-analog converter (DAC) to the terminal 410, and a negative-polarity voltage INN is inputted from a negative DAC to the terminal 420. The switch circuit 500 is provided with switches SW501 to SW504 to control connections between the differential input stage circuits 140 and 240 and the driving stage circuits 130 and 230. The switch circuit 600 is provided with switches SW601 to SW604 to control connections between the output terminals 110 and 210 and input terminals 121 and 221 of the differential input stage circuits 140 and 240.
The conventional operational amplifier can change the configuration of the operational amplifier circuit, which drives the odd terminal 310 and the even terminal 320, by the switch circuits 300 to 600. Specifically, a pattern 1 in which the switches SW301, SW303, SW401, SW403, SW501, SW503, SW601, and SW603 are set to on states while the switches SW302, SW304, SW402, SW404, SW502, SW504, SW602, and SW604 are set to off states, and a pattern 2 of the switch states opposite to the above-mentioned states are switched.
In a case of the pattern 1, the positive-polarity voltage INP from the positive DAC is inputted to the operational amplifier circuit formed from the differential input stage circuit 140 and the driving stage circuit 130, and an output from the output terminal 110 is outputted to the odd terminal 310 as an odd output Vodd. At this time, the negative-polarity voltage INN from the negative DAC is inputted to an operational amplifier circuit formed from the differential input stage circuit 240 and the driving stage circuit 230, and an output from the output terminal 210 is outputted to the even terminal 320 as an even output Veven.
On the other hand, in the case of the pattern 2, the positive-polarity voltage INP from the positive DAC is inputted to the operational amplifier circuit formed by the differential input stage circuit 240 and the driving stage circuit 130, and an output from the output terminal 110 is outputted to the even terminal 320 as an even output Veven. At this time, the negative-polarity voltage INN from the negative DAC is inputted to the operational amplifier circuit formed from the differential input stage circuit 140 and the driving stage circuit 230, and an output from the output terminal 210 is outputted to the odd terminal 310 as an odd output Vodd.
As described above, the conventional operational amplifier operates in the above-described manner to drive capacitive loads connected to the odd terminal 310 and the even terminal 320. At this time, the differential input stage circuits 140 and 240 and the driving stage circuits 130 and 230 operate within the voltage range from the high-voltage side power supply voltage VDD to the low-voltage side power supply voltage VSS, and the transistor MP180, the transistor MP280, the transistor MN180 and the transistor MN280, which are output transistors, operate within the voltage range from the high-voltage side power supply voltage VDD to the intermediate voltage VDD/2 or the range from the intermediate voltage VDD/2 to the low-voltage side power supply voltage VSS, respectively. Thereby, it is made possible to make power consumed in an output stage about half.
In the conventional operational amplifier, the effect that power consumption amount (especially, static power consumption amount) is reduced to about half is provided by such power supply connections as shown in FIG. 1. In the conventional operational amplifier, however, it is required to provide the switches for the polarity inversion on the output side of the output stage, as shown in FIG. 1. FIG. 2 is a diagram showing output waveforms simulated by using the sizes of the switches SW301 to SW304 as parameters in FIG. 1. As shown in FIG. 2, the characteristic varies greatly according to the sizes of the switches SW301 to SW304. When the sizes of the switches SW301 to SW304 are small, that is, when on resistances of the switches are large, the output waveform is dull. If such an operational amplifier is used for a driving amplifier of a liquid crystal display panel, insufficient writing into a liquid crystal pixel capacitor is generated, which causes image degradation. Therefore, it is required to increase the sizes of the switches in order to improve the characteristic. As a result, the size of the chip is increased, leading to cost rise.